Fuel cells are increasingly being used as a power source for electric vehicles and other applications. In proton exchange membrane (PEM) fuel cells, hydrogen is supplied to an anode catalytic electrode of the fuel cell and air with oxygen as an oxidant is supplied to a cathode catalytic electrode of the fuel cell. The electrochemical reaction that occurs between the reactant gases in the fuel cell consumes the hydrogen at the anode side and the oxygen at the cathode side and produces product water in liquid and vapor phase at the cathode side. PEM fuel cells include a membrane electrode assembly (MEA) with a thin, proton transmissive, non-electrically conductive, solid polymer electrolyte membrane joined with the anode catalytic electrode on one face and the cathode catalytic electrode on an opposite face.
Gas diffusion media play an important role in PEM fuel cells. Generally disposed between catalytic electrodes and the flow field channels of the bipolar plates in the fuel cell, the porous gas diffusion media provide reactant and product permeability, electrical conductivity, and thermal conductivity, as well as mechanical support for the soft MEA. Efficient operation of the fuel cell depends on the ability to provide effective water management in the system. In PEM fuel cells, the water management has to be carefully balanced to provide the proton exchange membrane with enough water to be sufficiently proton conductive and at the same time remove product water effectively in order to ensure that the gaseous reactants can access the catalytic electrodes without blockage by films or puddles of liquid water.
At dry operating conditions for which the unhumidified reactant gases are fed to the fuel cell, which is preferred to simplify the fuel cell system, product water mainly exists in vapor phase, i.e. occurrence of liquid water does not happen or only in small amounts. In this situation, the water vapor has to provide the source for membrane humidification in order to provide proton conductivity. The degree of humidity at the location of the membrane and catalytic electrode is closely related to the gas transport resistance of the gas diffusion medium since the vapor diffuses across the diffusion medium into the flow field channel. Consequently, high transport resistances are desired to keep the membrane humidified in dry situations by utilizing the product water vapor. In contrast to that, wet operating conditions (where the reactant gases are saturated with vapor and condensation can occur) provide water for membrane humidification in abundance, and low transport resistances for reactant access and water removal are required.
Typically, diffusion media used in PEM fuel cells have relatively constant transport resistance over the entire area of the media because the structure, size, and frequency of the pores in the diffusion media are uniform. Furthermore, the transport resistance of state-of-the-art materials is usually very low. The performance of automotive fuel cells using such current diffusion media is limited because reactant streams are often subsaturated with water vapor at the cell inlet but get increasingly saturated with vapor up to the point of condensation along the flow direction at the outlet. Hence, there is a large variation of humidity and current (i.e. water production) over the active area of the cell from flow inlet to outlet. Thus, a rate of product water removal in wet operating regions close to the cell outlet must be balanced with the need to retain water close to the cell inlet in order to maintain membrane hydration. The unresolved problem, hence, is to provide a fuel cell diffusion medium with varying transport resistance across the active area exhibiting high transport resistance at the cell inlet for water vapor retention at the membrane and low transport resistance at the outlet where water has to be removed effectively.
The gas transport resistance is defined as “f·h/Deff”, where “f” is a geometrical factor to account for land-channel geometry if the measurement of the gas transport resistance is done in a fuel cell configuration, “h” is the layer thickness, and “Deff” is the effective diffusion coefficient. The effective diffusion coefficient describes the diffusion coefficient of the gas species under consideration (e.g. water vapor) in the gas mixture (e.g. air) in the presence of a porous material. As on one hand the solid fraction in the porous material fills up a portion of the space that normally is accessible for diffusion and the diffusive flux (porosity effect), and on the other hand the pores usually are not straight across the porous material but inclined or wound thereby extending the path length (tortuosity effect), the effective diffusion coefficient naturally is smaller than the free diffusion coefficient. The effective diffusivity of porous materials is typically defined as Deff=D·ε/τ, where D is the free diffusion coefficient of the species in the mixture in absence of the porous material, ε is the porosity of the porous material (i.e. the ratio of the pore volume to the overall material volume) and τ is the tortuosity of the transport path in the pores of the porous material. Typically, the ratio of the free diffusion coefficient to the effective diffusion coefficient D/Deff is a quantitative measure for how far the porous medium constitutes an obstacle to the diffusion and diffusive flux. Derivation of the gas transport resistance term is described in the reference “D. Baker, C. Wieser, K. C. Nyerlin, and M. W. Murphy, “The Use of Limiting Current to Determine Transport Resistance in PEM Fuel Cells,” ECS Transactions, Vol. 3, pp. 989-999 (2006), hereby incorporated herein by reference in its entirety.
Since the layer thickness in fuel cell applications is limited due to technical restrictions, increasing the transport resistance by increasing the layer thickness is usually not possible. Hence, decreasing the effective diffusivity is required. This can be done by either decreasing the porosity ε, increasing the tortuosity τ, or both. Analytical studies have shown that porosity needs to be significantly reduced to show an effect, and furthermore the degree to which porosity can be reduced in current materials (which are random porous media based on carbon fiber papers with particulate coating made from carbon black) is limited, too. Hence, an attempt has been made to increase the tortuosity of gas diffusion media in a controlled manner.
Accordingly, the present invention is a diffusion medium adapted to provide varying local water management capability to enable optimized fuel cell performance. In the diffusion medium described herein, the diffusive gas transport resistance of the diffusion medium is maximized by varying the structure, size, frequency and arrangement of the pores particularly aiming at increasing the tortuosity, thereby maximizing an amount of water vapor retained in the PEM for hydration. Simultaneously, the transport resistance of the diffusion medium is localized and controlled across the diffusion medium by varying the structure, size, frequency and arrangement of the pores therein particularly aiming at controlling the tortuosity to selectively vary the transport resistance and tortuosity of the diffusion medium.